1. The Field of the Invention
This invention relates to devices and methods for distinguishing between different kinds of fluids and, more particularly, to a novel device and method for distinguishing between liquids and gases on the basis of their differing dielectric properties. This invention is particularly, but not exclusively, useful as an air-in-line detector for an IV infusion device.
2. The Background Art
Various kinds of fluid systems are currently in wide use in industrial, medical and many other applications. As used herein, the term "fluid" refers generally to any substance which is not solid and which is capable of flowing through a tube or conduit and, thus, includes both gases and liquids.
In fluid systems, it is often desirable to be able to distinguish between different fluids, such as, for example, water and air. Such detection is essential for the proper operation of many fluid systems.
For example, a fluid system may be intended to convey a liquid through a conduit. If sufficient quantities of air enter the conduit, however, the system may malfunction. For instance, if the system includes a pump for conveying the liquid through the conduit, the pump may cease to operate properly in the presence of sufficient quantities of air. The intended function of the system will, in any event, not be served unless the air is detected and removed from the system.
In fluid systems which are intended for medical applications, the early detection of an air bubble in a column of liquid can be vital. For example, a patient is often supplied with medication or other essential liquids through a tube which is connected to the patient through an intravenous (IV) catheter. If air bubbles enter the tube and are conveyed to the patient, the patient can be subjected to significant discomfort. Such air bubbles can in some cases even become life-threatening.
In order to insure the proper operation of fluid systems, especially those intended for medical use, those skilled in the art have attempted to develop devices that will automatically detect an air bubble in a column of liquid. Unfortunately, however, the prior art devices have typically been somewhat limited in their application, such as, for example, being limited to use with either an opaque liquid or a substantially transparent liquid. Those prior art systems which have been intended for use with both transparent liquids and opaque liquids have generally been quite complex and expensive, and also somewhat unreliable.
One type of system which is commonly used to detect the presence of an air bubble in a column of liquid uses a light source which is directed through the column. A photocell or some other light sensitive device is positioned adjacent the fluid column opposite the light source. Then, by detecting the intensity of the light transmitted through the column, the nature of the fluid in the column is ascertained.
For example, if an opaque liquid is being conducted through a transparent conduit, the absence of light transmitted through the conduit is indicative of liquid being in the column. If an air bubble enters the conduit, however, the fluid column will suddenly become transparent. This can be readily detected with the photocell and communicated to appropriate control circuitry.
When the liquid being conveyed through a transparent conduit is also substantially transparent, however, detecting an air bubble in the conduit becomes somewhat more challenging. In such cases, prior art detection devices are often based upon the different light transmission properties of air and liquid. For instance, liquid will generally cause a light beam to be refracted as it passes through the conduit, while air will not. Thus, if the photocell is carefully positioned and shielded so as to detect only a refracted light beam, the detection of light by the photocell is an indication that clear liquid is in the conduit. On the other hand, when such a properly positioned and shielded photocell does not detect a significant amount of light, it is likely that air or an opaque fluid is in the conduit.
It will be readily appreciated that the above-described devices for detecting the presence of an air bubble in a column of liquid are quite complex. These devices require careful alignment and/or shielding of the light source and the photocells. Additionally, when these detection devices are to be used with both opaque and transparent liquids, some means must be provided for initially detecting whether a given liquid is either opaque or transparent. This again adds complexity and cost to the devices and may tend to make their performance somewhat unpredictable.
In addition to photo-optical systems, various ultrasonic devices have also been proposed for the detection of an air-in-line condition. Ultrasonic devices, unlike the photo-optical devices, are not affected by fluid opacity. Instead, they depend on differences in the ultrasonic transmissive properties of the fluid in the tube to distinguish between whether a liquid or a gas is flowing through the tube. Ultrasonic air-in-line detection devices, like sophisticated photo-electric devices, however, are relatively more expensive to manufacture.
The present invention recognizes that an air-in-line condition can be detected by means that are reliable yet less expensive than the optical or ultrasonic systems. In accordance with the present invention, an air-in-line condition can be detected by effectively incorporating the fluid tube as a component of an electrical circuit. Specifically, the present invention takes advantage of the capacitive changes between an air-in-line condition and a liquid-in-line condition to determine when there is an unwanted air-in-line condition.